(1) Field of the Invention
The present invention generally relates to a system for removing sulfur oxides, other acid gases, particulate, and mercury from the flue gas of a fossil fuel fired combustor. In particular, the present invention is directed to an integrated dry/wet flue gas cleaning system.
(2) Description of the Related Art
Fossil fuel fired combustors and the like can generate large quantities of sulfur oxides and other acid gases. The sulfur oxides are emitted into the atmosphere through the flue gases from the combustors. The combustion process converts naturally occurring sulfur in the coal to gaseous sulfur dioxide (SO2), a criteria pollutant and precursor to acid rain, and sulfuric acid mist formed by condensation of sulfur trioxide (SO3), a precursor to PM2.5 and cause of visible emissions. PM2.5 refers to particulate matter that is 2.5 micrometers or smaller in size. Fine particles are of concern because they are risk to both human health and the environment. Other undesirable acid gas pollutants such as hydrogen chloride (HCl) and hydrogen fluoride (HF) may also be produced.
Clean and environmentally sound power generation and waste incineration requires economical air pollution control systems. Air pollution control systems are sometimes complex, and typically consist of stages for the removal of particulate, acid compounds, organic substances, heavy metals, as well as the disposal of by-products from these processes.
Two process types currently used to remove sulfur oxides from flue gas are wet flue gas desulfurization (WFGD) and dry flue gas desulfurization (DFGD). In WFGD, the flue gas enters a large vessel, e.g., a spray tower or absorber, which is generally referred to as a wet scrubber, where it is sprayed with an aqueous slurry, e.g., a mixture of water and at least partially insoluble matter, e.g., an alkaline matter such as lime, limestone, or the like. The calcium in the slurry reacts with the SO2 to form calcium sulfite or calcium sulfate. The calcium sulfite and/or sulfate is dewatered by various means to produce a solid by-product. When the by-product is primarily calcium sulfite, it is usually mixed with fly ash and fixative lime and disposed of in landfills. Alternatively, salable gypsum can be produced from the WFGD waste product by injecting compressed air in the wet scrubber.
In DFGD, a water slurry, e.g., water mixed with quicklime to form calcium hydroxide or similar, is introduced into a spray dryer tower. The slurry is atomized and injected into the flue gases where droplets react with SO2 as they evaporate in the vessel. The resulting dry waste product is collected in the bottom of the spray dryer and in particulate removal equipment, e.g., an electrostatic precipitator (ESP) or bag filter. Typically, the dry waste product is collected from the particulate removal equipment and disposed of in landfills.
WFGD typically has high capital costs due to the use of expensive corrosion resistant materials and extensive reagent and by-product handling systems. WFGD systems typically produce a liquid purge stream, which must be treated prior to disposal, and may produce a sulfur trioxide (SO3) acid mist emission, which is a pollutant that results in objectionable visible emissions and is a precursor to PM2.5. With existing WFGD technology, the SO3 mist must be eliminated by costly means such as Wet Electrostatic Precipitators (WESP) or alkali injection. Alternative desulfurization methods such as ammonia scrubbing are available, but are generally not economically competitive with existing wet and dry methods.
DFGD may be expensive to operate due to the relatively inefficient use of costly lime reagent and may create a solid waste disposal problem. The present dry sulfur removal methods generally fail to alleviate issues such as low percentages of sulfur oxide removal and poor reagent utilization. Often, spray drying is sensitive to operating conditions, making it difficult to maximize results. Depending on the amount of oxides present, the temperature must be adjusted precisely to create the desired reaction. Because the temperature must be maintained in a narrow range, the performance of the process is typically reduced. DFGD systems do, however, have the advantage of high SO3 removal efficiency thus avoiding problems stated above related to acid mist emissions.